It depends on your goals and constraints. I mentor a lot of undergraduate and high school student research. But I am flexible in that I am willing to mentor research in ANY field of the student's interest, not just my specialty. I am also flexible in that I can work with students without them relocating to be able to work in my lab.
I sort of recruit in some home schooling forums, but mostly I just keep my eyes open. One student, I met here on PF. He as asking some detailed questions about a science project, and we struck up a conversation by PM. His math project won 1st place last year in his regional science fair. I also attended a state science fair last year and gave some students my email address which has resulted in some collaborations.
A few other high school and college students have reached out to me via email looking for a mentor for senior research projects or just looking for help. Once I realize they have talent and are willing to work hard, I extend an offer of mentorship. I've reached out to one local high school to mentor some student
science projects. They seemed interested at first, but then kinda went silent.
Some high school students I've worked with have moved on to college and are actively seeking faculty for research opportunities. One found a great opportunity because they were a FAFSA work study student, and a lot of campus research jobs post in the work study program. If your school has a work study program, you definitely want to post there. Many schools also have an undergraduate research office which works to connect faculty with interested students. If your school has a Society for Physics Students, I bet the faculty adviser can put you in touch with some promising students.
But I should caution that most high school and undergraduate students are more of a time sink than a time saver. By the time they come up to speed and are actually capable of saving you time and energy on projects, they will soon be off to the next phase of their lives. So I tend to end up putting more into most students than I ever get out of them. I look at it as investing in the next generation of scientists rather than reducing my labor. I get them trained up and capable, and then they move on to reduce someone else's labor. I'm OK with that.